The present invention relates to a drilling tool and, more particularly but not limitatively, to a drilling tool or bit attachable to a lower end of a drilling string, with the tool having several revolving parts fitted with cutting elements, whereby these elements may be conical wheels revolving on bearings whose axes are inclined with respect to the central axis of the tool.
In the past, the performance of such tools, used for drilling into the ground, has been improved by the simultaneous action of jets of drilling fluid which strike the bottom of the hole or working face, in each of the free spaces between the elements cutting the ground. At the tool level, the essential purpose of this drilling fluid is to cool the tool, clean the tool and working face, and rapidly evacuate the cuttings to the annular space between the drilling string and the wall of the drilled hole.
In a first type of tool according to the prior art, the jets of fluid emerge at a point well above the cutting level of the wheels. As a result, before reaching the working face, the jets pass through the cutting-laden drilling fluid which fills the bottom of the drilled hole. For this reason, the flowrate of the jets at the level of the working face is considerably slowed down, which reduces their efficiency. Moreover, the jets sweep some of the cutting-laden drilling fluod to the working face where the cuttings are re-ground by the tool whose efficiency is thus decreased. Moreover, the jets of fluid create an overpressure at the working face which compresses the rock, and it has been found that the contact zone between the tool and the working face where the cuttings are produced is insufficiently irrigated by the drilling fluid.
Various modifications have been proposed and, in particular, tools of the type indicated above have been modified so that the jets of drilling fluid emerge as close as possible to the working face and sometimes even an axial jet has been added. However, these improvements have not given full satisfaction since a re-grinding of the cuttings could not be eliminated, nor could the overpressure at the working face.
In a second type of tool according to the prior art, it has been proposed to use, in combination with the irrigation means comprising fluid jets, means 4 aspirating the cutting-laden fluid, which means comprises a jet directed in the direction opposite the tool advance direction.
Tools of the aforementioned type are disclosed in, for example, French Patents 2,378,938, 2,421,270, and 2,421,271.
The present invention proposes a tool of the first type defined above which by design is simple and rugged in construction while having a markedly enhanced application performance by comparison with tools of the same type, approaching that of tools of the second type while not having aspiration jets.
In accordance with advantageous features of the present invention, a ground drilling tool is provided having a tool body rotationally driven by a tool holder, with a cavity being provided in the tool body for receiving a pressurized fluid through the tool holder. A plurality of revolving parts in the form of rollers or conical wheels are provided and are carried by the tool body, with the revolving parts being provided with teeth along a working face thereof for biting into the ground. Irrigation means deliver at least one jet of irrigation fluid directed toward the working face, with the irrigation means comprising at least a first calibrated orifice in the tool body, communicating with the cavity of the tool body and opening into a space between two adjacent revolving parts. The calibrated orifice produces a jet of fluid directed at one of the two revolving parts between which the first space is defined, with the center of the jet of fluid at the outlet of the orifice being located at a distance from a plane P perpendicular to the tool axis passing through the tips of the teeth as viewed from the working face. The magnitudes of the tool diameter, height of the teeth, and distance of the outlet are determined in accordance with the following relationship: EQU d&gt;1.8H and EQU 0.125D&lt;d&lt;0.185D,
where:
d=a distance of the outlet from a plane P perpendicular to the tool axis;
H=a height of the teeth; and
D=a diameter of the tool.
When the tool according to the invention has irrigation means which comprise an extension containing the calibrated orifice, the extension being at a minimum distance t from the plane P, then the magnitudes D, H, and t having the following relationship: EQU t&gt;1.5H and EQU 0.1D&lt;t&lt;0.15D.
The irrigation means may comprise at least two calibrated orifices for producing jets of fluid directed at the two revolving parts between which the first space is defined.
The irrigation means may have at least one flow nozzle or irrigation nozzle containing the calibrated orifice or orifices.
The jet of fluid produced by the orifice may have an orientation essentially tangential to the outer surface of the revolving parts.
The revolving parts may have essentially a conical shape, and the orifice may be composed of elongated slots whose elongation direction may be essentially parallel to a generatrix of a revolving part.
The tool according to the invention may have three revolving parts defining three spaces between two adjacent revolving parts, of which only two spaces may be equipped with calibrated orifices, with only one of these spaces having no fluid injection orifice.
The axis of the jet produced by the orifice, when the latter is supported by a nozzle, may make an angle of between 15.degree. and 45.degree. with plane P, and preferably an angle equal to 30.degree..
When the injection orifice is slot-shaped, two of the walls defining the internal shape of this slot may lie in two planes making an angle of 15.degree. between them.
According to one version of the present invention, the tool may have two calibrated orifices located on either side of a plane essentially parallel to the tool axis, essentially containing the axis of rotation of a revolving part. These orifices may produce one jet each, one oriented toward the revolving part and the other directed at the working face. Moreover, these orifices may produce jets offset with respect to each other.
The tool may have two other calibrated orifices, with the orifices producing one jet directed toward the working face and one toward one of the other revolving parts, respectively.
Each of the jets produced by the other orifices may be oriented essentially at 60.degree. with respect to the median axis defined by the two revolving parts closest to the orifice.
One of the offset jets may make an angle of approximately 30.degree. with the median axis defined by the two revolving parts closest to this orifice.
The other offset jet may subtend an angle .lambda. which is approximately determined in accordance with the following relationship: ##EQU1## where:
D=the diameter of the tool; and
e=a distance separating a center of a calibrated orifice from a circumference of the tool.
The tool according to the present invention is particularly suitable for drilling soft, sticky ground.